Poly(isobutylene-co-isoprene) or IIR, is a synthetic elastomer commonly known as butyl rubber (or butyl polymer) which has been prepared since the 1940's through the random cationic copolymerization of isobutylene with small amounts of isoprene (usually not more than 2.5 mol %). As a result of its molecular structure, IIR possesses superior air impermeability, a high loss modulus, oxidative stability and extended fatigue resistance.
Butyl rubber is understood to be a copolymer of an isoolefin and one or more, preferably conjugated, multiolefins as comonomers. Commercial butyl comprises a major portion of isoolefin and a minor amount, usually not more than 2.5 mol %, of a conjugated multiolefin. Butyl rubber or butyl polymer is generally prepared in a slurry process using methyl chloride as a diluent and a Friedel-Crafts catalyst as part of the polymerization initiator. This process is further described in U.S. Pat. No. 2,356,128 and Ullmanns Encyclopedia of Industrial Chemistry, volume A 23, 1993, pages 288-295.
Halogenation of this butyl rubber produces reactive allylic halide functionality within the elastomer. Conventional butyl rubber halogenation processes are described in, for example, Ullmann's Encyclopedia of Industrial Chemistry (Fifth, Completely Revised Edition, Volume A231 Editors Elvers, et al.) and/or “Rubber Technology” (Third Edition) by Maurice Morton, Chapter 10 (Van Nostrand Reinhold Company (c) 1987), pp. 297-300.
The development of halogenated butyl rubber (halobutyl, or XIIR) has greatly extended the usefulness of butyl by providing much higher curing rates and enabling co-vulcanization with general purpose rubbers such as natural rubber and styrene-butadiene rubber. Butyl rubber and halobutyl rubber are high value polymers, as their unique combination of properties (for example, excellent impermeability, good flex, good weatherability, co-vulcanization with high unsaturation rubbers, in the case of halobutyl) make them preferred materials for various applications, such as their use in making tire inner tubes and tire inner liners.
Like other rubbers, for most applications, butyl rubber must be compounded and vulcanized (chemically cross-linked) to yield useful and durable end products.
Reactive extrusion is sometimes used for making graft copolymers at commercial scale. This technology typically employs high process temperatures; however, previous research has shown that, at elevated temperatures (>140° C.), halobutyl rubbers XIIR are known to decompose as a result of HX elimination accelerating β-scissions of allyl cation intermediates, which leads to fragmentations.
This thermal decomposition, as described by Parent et al in Macromolecules 2002, 35, 3374-3379, renders, for example, BIIR incompatible with extrusion conditions necessary to graft polyamide onto butyl rubber. As a result, it ultimately precludes halobutyl rubbers, notably bromobutyl rubbers from being used in high-temperature compounding.
There exists prior art relating to ionomer formation from halogenated butyl polymer. For example, R. Resendes et al. in US20090299000 teach how halogenated butyl polymer is converted into an ionomer using N- and P-nucleophiles. The preparation of isobutylene-based ionomers through displacement of halide from brominated poly(isobutylene-co-isoprene) (BIIR) by triphenylphosphine (PPh3) and N,N-dimethyloctylamine (DMOA) is demonstrated J. S. Parent et al. in “Synthesis and Characterization of Isobutylene-based Ammonium and Phosphonium Bromide Ionomers,” Macromolecules 37, 7477-7483, 2004. R. Resendes et al. in US20100010140 discloses a peroxide curable rubber nanocomposite compound comprising a peroxide curative, a nanoclay and a high multiolefin halobutyl ionomer prepared by reacting a halogenated butyl polymer having a high mol percent of multiolefin with at least one nitrogen and/or phosphorus based nucleophile. The resulting high multiolefin halobutyl ionomer comprises from about 2 to 10 mol % multiolefin. The present invention is also directed to a shaped article comprising the rubber compound.
There also exists prior art relating to the grafting of non-halogenated amine-reactive compounds (i.e., maleic anhydride, glycidyl acrylate, glycidyl methacrylate) to halobutyl via Diels-Alder reaction, and/or the use of the product of these reactions in blends with polyamides.
For example, GB Patent 1589985 teaches how to convert a halogenated butyl rubber into a conjugated butyl rubber and then to react it with MAH or maleic imide in Diels-Alder reaction.
U.S. Pat. No. 3,646,166 teach how to use halogenated butyl rubber and form conjugated double bonds in its structure, then react it with MAH forming a graft polymer of butyl rubber with MAH and react this graft polymer with an amine in solution forming a final reaction product.
EP0361769A2 describes the grafting of the amine-reactive grafting materials MAH, acrylic acid, and glycidyl methacrylate to halobutyl and its subsequent use of the resulting functional butyl elastomer in blending with polyamides. EP0361769A2 teaches that chlorobutyl is converted to conjugated diene butyl in a solution process and subsequently reacted with MAH, acrylic acid, and glycidyl methacrylate in a reactive mixing process. EP0361769A2 further teaches the blending of said modified butyl elastomers with polyamides.
However, heat instability of these polymers make them unsuitable for use in reactive extrusion processes for bonding with polyamides, which typically employ high process temperature. In addition, these polyamide-halobutyl grafts do not exhibit commercially useful physical properties, particularly in the case of Mooney viscosity, ultimate tensile strength and ultimate elongation. Further, these polyamide-halobutyl grafts are not suitable for pelletization, which makes them difficult to store, transport and work with at a commercially relevant production scale.
As a result, there remains a need to develop an economical approach for the grafting of polyamide to halobutyl rubber.